New Zealand authorities were cutting holes in 300 whale carcasses on Monday, popping the dead animals "like balloons", to avoid them exploding as they decompose on Golden Bay after more than 600 whales became stranded.

Hundreds of rescuers managed to save around 400 pilot whales on the South Island beach on the weekend after one of New Zealand's largest whale strandings.

But hundreds of whales died on the beach and the Department of Conservation (DOC) cordoned off the bodies and urged the public to call them if they found whale carcasses that had floated off the beach and washed up on nearby shores.

"The area is currently closed to the public because of the risk from whales exploding," the conservation department said in a statement.

Workers in protective clothing would spend the day cutting holes in the whale carcasses, "like popping balloons" with knives and two metre (six feet) needles, to release internal gases that build up pressure, a DOC spokesman told local radio.

It would take several months for the bodies to decompose and turn into skeletons.

The surviving whales were last seen swimming six kms (four miles) offshore on Sunday evening, according to DOC.

Last Thursday a pod of about 400 whales became stranded, with a second pod of more than 200 whales stranded on Saturday.

The precise cause of the whale strandings was not known.

Beached whales are not uncommon on Golden Bay. Its shallow muddy waters confuse the whale's sonar, leaving it vulnerable to stranding by an ebb tide, according to marine environmental organisation Project Jonah.

Pilot whales are not listed as endangered, but little is known about their population in New Zealand waters.